Oxytocin (OT) and arginine vasopressin (AVP) are neuropeptides that play critical roles in social behavior, cognition, stress physiology, and physical health. Dysregulation of OT/AVP systems (alterations in peptide concentrations or the methylation of their receptor genes) has been implicated in adverse health outcomes including autism, schizophrenia, mood, anxiety, and personality disorders. OT is released in adult humans and dogs during affiliative forms of human-animal interaction (HAI), and HAI attenuates AVP release in dogs. Thus, HAI may provide a safe and effective approach for stimulating endogenous OT release, and inhibiting endogenous AVP activity. However, it is unknown 1) whether HAI similarly stimulates OT release in children, 2) whether HAI affects AVP release in humans (adults or children), 3) whether characteristics of the dog-child relationship affect these responses, and 4) how methylation of the OT receptor gene (OXTR) affects children?s attachment to pets, or their physiological responses to HAI. The objective of this proposal is to identify how OT and AVP systems contribute and respond to HAI in children, and to elucidate the relationships between these mechanisms and the psychosocial processes of HAI. We will recruit a sample of typically developing 8-10 year old children who will engage in structured HAI sessions with a familiar companion dog or unfamiliar dog, compared to a nonsocial control condition. In Aim 1 we will measure short-term changes in OT, AVP, and cortisol, in children engaging in HAI compared to a control condition. We hypothesize that through activation of neural networks that promote attention to social stimuli and encoding of social reward, OT facilitates and responds to forms of social engagement that provide a sense of safety, social support, and emotional connectedness during HAI. We also hypothesize that HAI attenuates AVP release in children, and that increases in OT coupled with decreases in AVP act to reduce hypothalamic-pituitary- adrenal activity. In Aim 2 we will measure short-term changes in OT, AVP and cortisol in dogs during interaction with children, and assess coordination between dogs? and children?s physiological responses. We hypothesize that HAI will generate increases in OT, and decreases in AVP and cortisol, in both children and dogs. We hypothesize that physiological responses will be coordinated between partners, and associated with the extent of affiliative social behavior between the child and dog. In Aim 3 we will investigate relationships between child OXTR methylation (a biomarker for OXTR expression in the brain) and children?s attachment to pets and behavior during HAI. We hypothesize that OXTR methylation will be negatively associated with children?s attachment to pets, as well as the extent of their social engagement with dogs during HAI sessions. Ultimately, this work will elucidate relationships between HAI and OT/AVP pathways, advancing our understanding of the biological mechanisms through which HAI affects child health and development.